Current animal models of pulmonary arterial hypertension (PAH) have shortcomings that limit understanding of PAH pathobiology and identifying approaches to reduce occlusion of pulmonary arteries. The current application shows findings that indicate that vascular endothelial stem cells (VESC) cause PAH through uncontrolled cell growth and endothelial-to-mesenchymal transition (EndoMT). The findings are that VESC transplantation combined with chronic hypoxia leads to occlusion of pulmonary arteries and severe PAH. This approach is here proposed as a novel VESC- focused animal model of PAH. Additional preliminary data suggest that VESC proliferate less without fibroblast growth factor-2 (FGF-2) and that VESC undergo EndoMT in the presence of transforming growth factor-? (TGF-?). Given these remarkable findings, the hypothesis is VESC cause PAH with vascular lesions resembling human PAH via two main means: unchecked proliferation driven by FGF-2 and EndoMT in the presence of TGF-?. Three specific aims are proposed to test the hypothesis: Aim 1: To define dynamic changes in physiology, histology and gene expression in VESC-induced PAH. Aim 2: To investigate the role of FGF-2 for unchecked growth of VESC in PAH. Aim 3: To identify the role of TGF-? for EndoMT of VESC in PAH. To achieve these aims, in vitro and in vivo experiments using clonally selected VESC will be used. First, a new model of VESC- induced PAH will be evaluated and the results will be compared with known physiology, histology and gene expression profile as identified in patients with PAH. Then, the role of FGF-2 in VESC proliferation will be evaluated by blocking FGF-2 in vitro and in vivo. Finally, the contribution of TGF-? to EndoMT for PAH will be testing by blocking TGF-? in in vitro and in vivo experiments. This study is innovative in two aspects: (1) this is the first study to investigate VESC as cause of unchecked cell growth and EndoMT in PAH; (2) these experiments will improve upon existing animal models by combining labeled VESC clones with chronic hypoxia. Preliminary data suggest that such a strategy will lead to a new and unique model of PAH. The strength of this model is its origin from a pathogenic concept for iPAH: clonal expansion of VESC causes occlusion of pulmonary arteries. It is the expectation that such studies will start to reveal why VESC fail to repair vessels and instead cause PAH. Such results are expected to help developing strategies to reduce PAH and at the same time to foster vascular repair by VESC. Such data are then expected to help finding new drug targets. Further research resulting from these data could reduce mortality in PAH patients. Hence, the proposed research is pertinent to developing fundamental knowledge that will help to reduce the burden of cardiopulmonary disease.